Hemoglobin-based oxygen carriers

The artificial oxygen carrier erythrocruorin-characteristics and potential significance in medicine

The diminishing supply and increasing costs of donated blood have motivated research into novel hemoglobin-based oxygen carriers (HBOCs) that can serve as red blood cell (RBC) substitutes. HBOCs are versatile agents that can be used in the treatment of hemorrhagic shock. However, many of the RBC substitutes that are based on mammalian hemoglobins have presented key limitations such as instability and toxicity. In contrast, erythrocruorins (Ecs) are other types of HBOCs that may not suffer these disadvantages. Ecs are giant metalloproteins found in annelids, crustaceans, and some other invertebrates. Thus far, the Ecs of Lumbricus terrestris (LtEc) and Arenicola marina (AmEc) are the most thoroughly studied. Based on data from preclinical transfusion studies, it was found that these compounds not only efficiently transport oxygen and have anti-inflammatory properties, but also can be modified to further increase their effectiveness. This literature review focuses on the structure, properties, and application of Ecs, as well as their advantages over other HBOCs. Development of methods for both the stabilization and purification of erythrocruorin could confer to enhanced access to artificial blood resources.

Modulation of oxidative stability of haemoglobin inside liposome-encapsulated haemoglobin

The major hurdle in the formulation of liposome-encapsulated haemoglobin (LEH) is the oxidation of haemoglobin (Hb) into methaemoglobin during storage and after administration. In order to reduce this oxidative degradation, we tested various reducing conditions in the presence of catalase. We found that at 37°C more than 50% of Hb oxidized to methaemoglobin within 24 h, whereas in presence of catalase, the oxidation was significantly reduced. The effect of catalase was further enhanced by a reduction mixture containing β-NAD, d-glucose, adenine, inosine, MgCl2, KCl, KH2PO4 and Na2HPO4; only 14% methaemoglobin was generated in the presence of catalase and reduction mixture. Contrary to the expectation, glutathione, deferoxamine and homocysteine enhanced Hb oxidation. The presence of CRM inside liposomes (250 nm) significantly decreased Hb oxidation. The results suggest that catalase and a well-defined mixture of co-factors may help control Hb oxidation for improvement in the functional life of LEH.

Use of liposome encapsulated hemoglobin as an oxygen carrier for fetal and adult rat liver cell culture

Engineering liver tissue constructs with sufficient cell mass for transplantation implies culturing large numbers of hepatocytes in a reduced volume; however, providing sufficient oxygen to dense cell cultures is still not feasible using only conventional culture medium. Liposome-encapsulated hemoglobin (LEH), an oxygen-carrying blood substitute originally designed for short-term perfusion, may be a good candidate as an oxygen carrier to cultured liver cells. In this study, we investigated the feasibility of maintaining long term hepatocyte cultures using LEH. Primary fetal and adult rat liver cells were directly exposed to LEH for 6 to 14 days in static culture or in a perfused flat plate bioreactor. The functions and viability of adult rat hepatocytes exposed to LEH were not adversely affected in static monolayer culture and were even improved in the bioreactor. However, some cytotoxicity of LEH was observed with fetal rat liver cells after 4 days of culture. LEH, though a suitable oxygen carrier for long-term culture of mature hepatocytes, is not suitable in its present form for perfusing fetal hepatocyte cultures in direct contact with the liposomes; either the LEH will have to be made less toxic or a more sophisticated bioreactor that prevents the direct contact between hepatocytes and perfusates will have to be designed if fetal cells are to be used for liver tissue engineering.

Magnetic resonance spectroscopy for evaluation of liposome-encapsulated hemoglobin as a resuscitation fluid

Liposome-encapsulated hemoglobin (LEH) based on a novel, synthetic, non-phospholipid was developed, and evaluated for cerebral energy metabolism in a 40% hemorrhage rat model. The markers of tissue energetics were monitored by (1)H- and (31)P-magnetic resonance spectroscopy (MRS). After hemorrhage, (1)H-MRS showed an increase in the levels of lactate and pyruvate. These markers returned to baseline values following LEH resuscitation. Both LEH and saline were able to exert a neuron-protective effect as indicated by the recovery of N-acetylaspartate. (31)P MRS showed a fall in phosphocreatine after hemorrhage, which upon LEH or saline resuscitation returned to the baseline values. Similarly, inorganic phosphate increased after bleeding, but returned to normal after resuscitation. LEH resuscitation also recovered beta-ATP levels, but saline resuscitation provided only a modest recovery. The results indicate the utility of MRS to monitor cerebral metabolism in hemorrhage/resuscitation. The data is also supportive of the new LEH formulation as an oxygen carrier.

Interference in clinical chemistry assays by the hemoglobin-based oxygen carrier, Hemospan

OBJECTIVE

To evaluate interference by the hemoglobin-based oxygen carrier Hemospan on clinical laboratory assays.

DESIGN AND METHODS

Interfering Hemospan concentrations were determined for general chemistry and cardiac marker analytes in pooled serum and the corresponding hemolysis index was calculated.

RESULTS

Hemospan did not interfere with 20 of 35 analytes. Hemospan produced a negative interference in serum creatinine, amylase, alkaline phosphatase, uric acid, and GGT assays and a positive interference in serum phosphate, LDH, iron, triglycerides, total protein, AST, cholesterol, magnesium, and albumin assays, and appeared to positively bias the serum cardiac troponin I (cTnI) assay only when cTnI is present in the sample.

CONCLUSIONS

We present a report of assays affected by Hemospan and the threshold concentrations for interference. This study highlights the importance of interference studies in understanding the effects of hemoglobin-based oxygen carriers on results reported by the clinical laboratory.

Cerebral oxygen delivery by liposome-encapsulated hemoglobin: a positron-emission tomographic evaluation in a rat model of hemorrhagic shock

Liposome-encapsulated Hb (LEH) is being developed as an artificially assembled, low-toxicity, and spatially isolated Hb-based oxygen carrier (HBOC). Standard methods of evaluating oxygen carriers are based on surrogate indicators of physiology in animal models of shock. Assessment of actual delivery of oxygen by HBOCs and resultant improvement in oxygen metabolism at the tissue level has been a technical challenge. In this work, we report our findings from 15O-positron emission tomographic (15O-PET) evaluation of LEH in a rat model of 40% hypovolemic shock. In vitro studies showed that PEGylated LEH formulation containing approximately 7.5% Hb and consisting of neutral lipids (distearoylphosphatidylcholine:cholesterol:alpha-tocopherol, 51.4:46.4:2.2) efficiently picks up 15O-labeled oxygen gas. The final preparation of LEH contained 5% human serum albumin to provide oncotic pressure. Cerebral PET images of anesthetized rats inhaling 15O-labeled O2 gas showed efficient oxygen-carrying and delivery capacity of LEH formulation. From the PET images, we determined cerebral metabolic rate of oxygen (CMR(O2)) as a direct indicator of oxygen-carrying capacity of LEH as well as oxygen delivery and metabolism in rat brain. Compared with control fluids [saline and 5% human serum albumin (HSA)], LEH significantly improved CMR(O2) to approximately 80% of baseline level. Saline and HSA resuscitation could not improve hypovolemia-induced decrease in CMR(O2). On the other hand, resuscitation of shed blood was the most efficient in restoring oxygen metabolism. The results suggest that 15O-PET technology can be successfully employed to evaluate potential oxygen carriers and blood substitutes and that LEH resuscitation in hemorrhage enhances oxygen delivery to the cerebral tissue and improves oxygen metabolism in brain.

Toxicity and hemodynamic effects after single dose administration of MalPEG-hemoglobin (MP4) in rhesus monkeys

Maleimide-polyethylene glycol-modified (MalPEG) hemoglobin, 4.3 g/dL (MP4; Hemospan), is a hemoglobin-based oxygen carrier consisting of human hemoglobin (Hb) modified with maleimide polyethylene glycol. This study evaluates the potential toxicity and hemodynamic actions of a single dose of MP4 administered by exchange transfusion to rhesus monkeys. Monkeys were administered MP4 (21 mL/kg, or approximately 30% of estimated blood volume) or an equivalent volume of lactated Ringer's solution (LR). In the toxicity study, blood samples were obtained predose and 3, 7, and 13 days after dosing for clinical chemistry and hematology. Animals were euthanized for complete necropsy and histopathology on day 3 or day 13. A separate group of animals was used for evaluation of arterial pressure, core temperature, and electrocardiogram, by telemetry, for 7 days after dosing with MP4. The results demonstrate no significant toxicity, with only modest, transient elevation of aspartate aminotransferase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase (LDH) on day 3. Mild anemia caused by hemodilution was observed at each time point in both groups, but to a slightly greater degree in the MP4-treated animals. Histologic observations included foamy or vacuolated macrophages in the spleen and marrow of the sternum, rib, and femur, representing the accumulation of test article or a metabolite. In the telemetry study, no changes occurred in arterial pressure, heart rate, or electrocardiogram attributable to administration of MP4 at any time for 7 days after administration. These results demonstrate that MP4 is safe and is without hemodynamic effects when administered as an exchange transfusion of 30% of estimated blood volume.

High O2 affinity hemoglobin-based oxygen carriers synthesized via polymerization of hemoglobin with ring-opened 2-chloroethyl-β-D-fructopyranoside and 1-o-octyl-β-D-glucopyranoside

Second generation hemoglobin-based O(2) carriers (HBOCs) are being developed with high O(2) affinity (low P(50)) in order to suppress vasoconstriction elicited by over-oxygenating tissues, a problem associated with low O(2) affinity first generation HBOCs. Our group has previously investigated the polymerization of hemoglobin (Hb) with dialdehydes as a strategy for engineering high O(2) affinity HBOCs. In this study, two novel reactive dialdehydes were synthesized by ring-opening 2-chloroethyl-beta-D-fructopyranoside (2-CEFP) and 1-o-octyl-beta-D-glucopyranoside (1-OGP) at the 1,2-diol position, respectively, to yield novel Hb polymerizing reagents. High-affinity polymerized HBOCs were synthesized by reacting R-state bovine hemoglobin (bHb) with ring-opened 2-CEFP and 1-OGP at cross-linker to bHb molar ratios ranging from 10:1 to 30:1. The resulting polymerized bovine HBOCs (bHBOCs) displayed P(50)s ranging from 7 to 18 mmHg, cooperativities ranging from 0.8 to 1.4, and methemoglobin (metHb) levels ranging from 3% to 10%. The cross-linking reaction also stabilized the third stepwise Adair coefficient for bHbs reacted with ring-opened 1-OGP at cross-linker to bHb molar ratios of 20:1 and 30:1 and for bHbs reacted with ring-opened 2-CEFP at molar ratios of 30:1. Additionally, the number-averaged molecular weight, M(n), of each polymerized bHBOC was larger compared to bHb. Molecular weight distributions leaning towards larger molecular weight bHBOCs were obtained by increasing the cross-linker to bHb molar ratio. Taken together, the results of this study have identified novel Hb polymerization reagents that are easy to synthesize, and that are capable of yielding bHBOCs with higher O(2) affinities and weight-averaged molecular weights compared to bHb.

Oxygen Therapeutics: Oxygen Delivery Without Blood

Nearly 14 million units of packed red blood cells are transfused in the United States each year. According to the U.S. Department of Health and Human Services, in 1999, 6% of hospitals reported a shortage of blood, resulting in the cancellation or postponement of surgical procedures. The many limitations and risks of transfusions of packed red blood cells in critically ill patients have facilitated interest in developing alternative agents for oxygen delivery. Over the past few decades, safe and effective substitutes have been in development. However, no currently approved agent provides both oxygen transport and volume in place of packed red blood cells. Oxygen therapeutic products have several advantages compared with packed red blood cells, including a prolonged shelf-life, lack of a cross-matching requirement, and minimal infectious risks or concerns about immunogenicity. Hemoglobin-based oxygen carriers and perfluorocarbons are being developed. Two products are undergoing clinical trials. Polyheme is undergoing a phase III study in trauma patients, and Hemopure is being evaluated in a phase II study in patients undergoing cardiopulmonary bypass surgery. A third product (Hemolink) was being evaluated in a phase III study in patients undergoing coronary artery bypass grafting surgery; however, the trial was suspended. In addition, several other hemoglobin-based oxygen carriers are in the preclinical stages. Oxygen therapeutics have several potential clinical applications in the management of perioperative blood loss, trauma, acute normovolemic hemodilution, traumatic brain injury, and blood requirements in patients who refuse or have contraindications to transfusions of red blood cells.

Can the kidney function as a lung? Systemic oxygenation and renal preservation during retrograde perfusion of the ischaemic kidney in rabbits

OBJECTIVE

To investigate renal preservation by a novel method of perfusion using an oxygenated perfluorocarbon (PFC) emulsion via retrograde access to the kidney, as preserving renal function during urological surgery has been elusive, and the recognized technique of nephron-sparing surgery has increased its application and practice in modern urology.

MATERIALS AND METHODS

After institutional review and approval, 30 New Zealand White rabbits were studied. In a solitary kidney model, each rabbit had the ureter catheterized before 40 min of renal artery occlusion. Each rabbit was randomized to one retrograde perfusion group, i.e. sham, normothermic PFC, chilled PFC, normothermic saline, and chilled saline. The rabbits were maintained for 2 weeks, during which renal function, urine output, systemic blood gases, weight and serum creatinine level were measured. After death, the kidneys were individually examined and graded by one renal pathologist unaware of the treatment.

RESULTS

The rabbits treated with retrograde PFC perfusion (normothermic and chilled) had less change in their creatinine clearance, at 3.6 and 4.0 mL/min per kg, than the sham group, at 7.8 mL/min per kg, while also having significantly higher systemic venous oxygenation, at 26.3 and 10.0 mmHg, than the sham group, at 0.2 mmHg. Normothermic and chilled perfusion with PFC was also associated with less histological evidence of ischaemic damage, with mean (sd) scores of 13.0 (13.5) and 8.7 (4.5), respectively, than in the sham group, at 33.3 (16.8), while favourably matching the contralateral control kidney group, at 5.5 (2.3). The rabbits treated with saline retrograde perfusion also had better outcomes than the sham cohort. There were no adverse effects in any of the study arms or with the use of PFC.

CONCLUSION

Retrograde oxygen delivery to the kidney through the urinary collecting system was successful in this pilot study. Renal function, laboratory and histological data indicate a trend towards renal preservation and even systemic oxygenation in the experimental groups compared with the sham rabbits, with no adverse effects attributed to this technique.

Current status of oxygen carriers ('blood substitutes'): 2006

An alternative to blood transfusion, based on oxygen-carrying solutions, has been sought for over a century. The present 'first-generation' haemoglobin-products were based on observations that crosslinking with, for example, glutaraldehyde, overcame subunit dissociation and renal toxicity. Experience with these solutions has shown that they can be vasoactive, sometimes increasing blood pressure, sometimes decreasing tissue perfusion and sometimes both. Clinical trials have been disappointing because of unexpected toxicity. The 'second-generation' products are based on a better understanding of the mechanisms of this vasoconstriction. Such products may seem counterintuitive by traditional standards, but it is hoped that they will be less toxic, more beneficial to patients, and more economical to produce.

What happened to blood substitutes?

Concerns about the safety and adequacy of the blood supply have fostered twenty years of research into the so-called "blood substitutes" among them the oxygen carriers based on modified hemoglobin. Although none of these materials has yet been licensed for use in North America or Europe, the results of research and clinical trials have increased our understanding of oxygen delivery and its regulation. In particular, the examination of the basis for the vasoactivity observed with some of the hemoglobin based oxygen carriers has led to the insight that several colligative properties of hemoglobin solutions, such as their diffusion coefficient for oxygen, viscosity and colloid oncotic pressure, are important determinants of efficacy.

Physical properties and stability mechanisms of poly(ethylene glycol) conjugated liposome encapsulated hemoglobin dispersions

Liposomes encapsulating hemoglobin (LEHs) surface-conjugated with 2000 and 550 Da poly(ethylene glycol) (PEG) were produced via extrusion through 400, 200 and 100 nm pore diameter membranes in two types of phosphate buffer with different ionic strengths. The lipid bilayers were composed of dimyristoyl-phosphatidylcholine (DMPC), cholesterol, dimyristoyl-phosphoethanolamine-PEG (DMPE-PEG), dimyristoyl-phosphatidylglycerol (DMPG), and alpha-tocopherol (in a 43:40:10:5:2 mole ratio). N-acetyl-L-cysteine was coencapsulated in order to suppress hemoglobin (Hb) oxidation. Various physical properties of PEG-LEH dispersions were determined: size distribution, encapsulation efficiency, P50 (partial pressure of O2 where half of the oxygen binding sites are saturated with O2), cooperativity coefficient, and encapsulated methemoglobin (MetHb) level. In order to study the stabilization mechanism of these dispersions, the effective bending constant (KB) and the spontaneous radius of curvature (R0) of PEG-LEHs were extracted by fitting a mathematical model describing the size distribution of a liposome dispersion to the experimentally measured size distributions. We observed that liposome dispersions extruded in phosphate buffer (PB) were more monodisperse than liposomes extruded in phosphate buffered saline (PBS), and higher molecular weight PEG promoted the formation of narrower size distributions. Moreover, extrusion in PB and lipid conjugation with higher molecular weight PEG imparted higher bilayer rigidity (high KB), and stabilized the liposome dispersions by the spontaneous curvature mechanism, whereas the other liposome dispersions were stabilized by thermal undulations (low KB). The P50 and cooperativity coefficient of PEG-LEHs extruded in PBS and PB was comparable to that of human blood, and the encapsulated MetHb levels were less than 5%. The highest encapsulation efficiencies obtained were 27%-36% (82-109 mg Hb/mL) for LEH dispersions extruded in PBS and grafted with 2000 Da PEG. These dispersions yielded KBs' ranging from 7kT to 27kT, which indicated that these dispersions were stabilized by spontaneous curvature. Whereas the same lipid combination extruded in PBS, however, instead conjugated with 550 Da PEG resulted in KBs' ranging from 2 kT to 2.7 kT, which indicated that these dispersions were stabilized by thermal undulations. Thermal undulations permitted Hb leakage through the lipid bilayers, which in turn lowered the encapsulation efficiency to 1%-10.7% (3-32 mg Hb/mL). Taken together, the experimentally measured size distributions and encapsulation efficiencies of PEG-LEH dispersions can be readily explained through analysis of the magnitude of KB, which dictates the stability mechanism of the liposome dispersion.

Stability of liposome encapsulated hemoglobin dispersions

The demand for artificial blood substitutes in cases of elective surgeries, trauma, and civilian mass catastrophes increases every day. However, few studies have been done to characterize the mechanical stability of blood substitutes, especially liposome encapsulated hemoglobin (LEHb) dispersions. In this work, the stability of LEHb dispersions was investigated by fitting Jung et al.'s liposome size distribution model to experimentally measured LEHb size distributions [6] (produced via extrusion) using asymmetric flow field-flow fractionation coupled with multi-angle static light scattering. The effective bending constant (KB) and radius of curvature (R0) of each liposome dispersion were regressed from the size distribution fits. The model was found to be in agreement with the size distributions of LEHbs extruded through 400, 200 and 100 nm pore diameter membranes, but not in agreement with LEHbs extruded through 80 and 50 nm pore diameter membranes. Although the magnitude of KB fluctuated, we deduced a general trend for KB to decrease with decreasing pore diameter, and increasing initial Hb concentration. LEHbs extruded through 400nm pore diameter membranes were stabilized by the spontaneous curvature effect, while those extruded through 80 and 50 nm pore diameter membranes were mostly stabilized by thermal undulations, regardless of the initial Hb concentration. For LEHb dispersions extruded through 200 and 100 nm pore diameter membranes, there was a transition of stabilization mechanism from spontaneous curvature to thermal undulations with increasing initial Hb concentration. Taken together, these results suggest that moderate Hb encapsulation might actually impart better mechanical stability to LEHb dispersions extruded through 200 and 100 nm pore diameter membranes.

MalPEG-hemoglobin (MP4) improves hemodynamics, acid-base status, and survival after uncontrolled hemorrhage in anesthetized swine

OBJECTIVES

MalPEG-hemoglobin, 4 g/dL (MP4), is a hemoglobin-based oxygen carrier with a low hemoglobin concentration, low P50 (oxygen half-saturation pressure of hemoglobin), high colloid osmotic pressure, and high viscosity. This study evaluated resuscitation with MP4 in anesthetized swine hemorrhaged 250 mL by controlled withdrawal, followed by a 5-mm tear in the abdominal aorta.

DESIGN

Randomized, unblinded.

SETTING

Academic animal laboratory.

SUBJECTS

Anesthetized male and female Swedish Landrace pigs.

INTERVENTIONS

Four groups of pigs (n = 7 each) were randomized after hemorrhage by aortic tear to receive 250 mL of MP4, Ringer's acetate, 10% pentastarch, or 4 g/dL of stroma-free hemoglobin, followed by aortic repair and transfusion of 250 mL of autologous blood. End points were 20-hr survival, hemodynamic variables, and acid-base status.

MEASUREMENTS AND MAIN RESULTS

Measurements included continuous aortic, pulmonary arterial, and central venous pressures, cardiac output by thermodilution, arterial and venous blood gases; electrolytes; lactate; base excess; oxygen delivery, consumption, and extraction ratio; hematocrit; hemoglobin; and urine output. Body weight (24-27 kg) and hemorrhage volume (26-33 mL/kg) were similar in the four groups. The nadir of mean arterial pressure (22-28 mm Hg) and the increase in lactic acid (5-8 mEq/L) after hemorrhage were similar in all groups, indicating equivalent shock in the four groups. Survival was greatest in the MP4-treated animals (six of seven) compared with Ringer's acetate (two of seven), 10% pentastarch (one of seven), and stroma-free hemoglobin (two of seven) and was accompanied by an improved recovery of arterial pressure, cardiac output, and lactate. Total hemoglobin concentration was equivalent in all groups. Arterial pressure did not increase above baseline values, and systemic vascular resistance was unchanged following administration of MP4, indicating the lack of peripheral vasoconstriction. Mortality in Ringer's acetate, stroma-free hemoglobin, and 10% pentastarch treated animals was associated with deteriorating acid-base status, low urine output, and hyperkalemia.

CONCLUSION

These data demonstrate that restoration of oxygen delivery with a small volume of MP4 yields significant recovery from hemorrhage without systemic vasoconstriction.

Liposome-encapsulated actin–hemoglobin (LEAcHb) artificial blood substitutes

A new approach to enhance the circulation persistence of liposomes has been applied to develop liposome-encapsulated actin-hemoglobin (LEAcHb) dispersions as potential blood substitutes by introducing an actin matrix into the liposome aqueous core. Asymmetric flow field-flow fractionation coupled with multi-angle static light scattering was used to study the shape, size distribution, and encapsulation efficiency of liposome-encapsulated hemoglobin (LEHb) and LEAcHb dispersions. By polymerizing monomeric actin into filamentous actin inside the liposome aqueous core, LEAcHb particles transformed into a disk-like shape. We studied the effect of an encapsulated actin matrix on the size distribution, hemoglobin (Hb) encapsulation efficiency, oxygen affinity, and methemoglobin (MetHb) level of LEAcHb dispersions, and compared them with plain LEHb dispersions (without actin). LEHb, and LEAcHb dispersions extruded through 400 nm membranes were injected into rats and it was observed that LEAcHb dispersions with 1mg/mL of actin enhanced the circulatory half-life versus LEHb dispersions. The circulatory characteristics of empty PEGylated and non-PEGylated actin-containing liposomes (without Hb) were studied as controls for the LEHb and LEAcHb dispersions in this paper, which displayed maximum circulatory half-lives greater than 72 h. Taken together the results of this study supports our hypothesis that a lipid membrane supported by an underlying actin matrix will extend the circulatory half-life of LEHb dispersions.

Lactate Measurement Interference by Hemoglobin-Based Oxygen Carriers (Oxyglobin??, Hemopure??, and Hemolink???)

We sought to determine whether hemoglobin-based oxygen carriers (HBOCs), hemoglobin glutamer-200 [bovine] (HBOC-200, Oxyglobin), hemoglobin glutamer-250 [bovine] (HBOC-201, Hemopure), and hemoglobin raffimer (Hemolink) interfere with the accuracy of lactate measurements. Combinations of concentrated L-lactate solution, HBOC, and blood or plasma with added PlasmaLyte-A were added to sample tubes to make a linear and constant increase in lactate concentration in consecutive samples. Sample lactate concentrations ranged from 5-110 mg/dL (0.6-12 mm) (physiological reference range: 5-20 mg/dL [0.56-2.2 mm]). Comparisons were made between machine measured lactate concentrations and calculated lactate concentrations. For Hb glutamer-250, the average difference between measured and calculated lactate concentrations was -5.1 mg/dL (-0.57 mm) (LX-20), with greater underestimation at larger lactate concentrations. For Hb raffimer, the average difference was -2.2 mg/dL (-0.24 mm) (LX-20). The veterinary product, Hb glutamer-200, was tested on 3 analyzers (LX-20(R), YSI 1500, and YSI 2300). The YSI 1500 was the most accurate instrument with the mean difference between measured minus calculated lactate being +1.3 mg/dL versus -2.6 mg/dL (YSI 2300) and -8.4 mg/dL (LX-20). The clinical implications of this study are that with increasing levels of an HBOC in plasma, lactate interpretation may become inaccurate, especially at larger lactate concentrations, causing underestimation of measured lactate values and possible under-treatment of the patient. Therefore, caution must be exercised when interpreting lactate results when a HBOC is present in plasma.

Functional characterization of the purified holo form of hemoglobin I from Lucina pectinata overexpressed in Escherichia coli

The clam Lucina pectinata inhabits the sulfide-rich west coast of the island of Puerto Rico. It contains three different hemoglobins. Hemoglobin I (HbI), which is monomeric at all concentrations, carries H2S in its ferric state. Overexpression of recombinant HbI from Lucina pectinata in BL21STAR(DE3) Escherichia coli cells was performed in the presence of delta-aminolevulinic acid (delta-ALA). Purification of the protein was achieved using FPLC anion exchange and size exclusion chromatography. Functional characterization of the recombinant holo-protein was assessed by detection of the protein heme O2, CO, and H2S derivatives by UV-Vis spectroscopy, with Soret maxima at 416, 421, and 425 nm, respectively. The results indicated that the recombinant HbI binds H2S and forms a heme sulfide complex like the HbI wild-type hemoglobin. Kinetic measurements were performed to determine the H2S affinity of the recombinant HbI. The H2S dissociation and association rate constants were 0.055 x 10(-3)s(-1) and 0.068 x 10(5) M(-1)s(-1), respectively. The H2S affinity constant of the recombinant HbI (0.124 x 10(9) M(-1)) is eightfold lower than that of the native clam HbI reported earlier. This effect is attributed mostly to the first of two missense mutations [Met 61 (E4)-->Val 61 and Ile101 (FG4)-->Val 101] and additional amino acids present in our construct as demonstrated by measurements of the association rate with a new construct lacking most of the additional residues and the missense mutations. The elimination of these residues restores the similarity between the expressed and wild-type hemoglobins, as evidenced by H2S association kinetics. A pH dependence on the H2S association rate was also contributing to the overall affinity constant and was taken into account in the measurements of the functional properties of the new HbI construct.

Kinetics of liposome-encapsulated hemoglobin after 25% hypovolemic exchange transfusion

Liposome-encapsulated hemoglobin (LEH) is being developed as an oxygen therapeutic. In this work, we evaluated a neutral formulation of PEGylated LEH for its circulation and distribution properties in rodent models of 25% hypovolemic exchange transfusion. About 25% of blood in rats and rabbits was exchanged with LEH that had been previously labeled with 99mTc radionuclide. The distribution of 99mTc-LEH was followed by gamma camera imaging and intermittent blood sampling during 48 h, and counting the tissue-associated radioactivity after necropsy at 48 h. On the basis of circulation kinetics, the half-life of 99mTc-LEH in blood was 30 and 39.8 h in rats and rabbits, respectively. Apart from blood, major organs of accumulation of LEH after 48 h included liver (rats, 10.3% and rabbits, 5.4% of injected dose) and spleen (rats, 2.4% and rabbits, 0.8% of injected dose). The results demonstrate that LEH circulates for a prolonged time after administration and that the animals tolerate at least 25% of blood exchange without any distress. Subsequent to the enhanced uptake in the RES, the rats clear LEH from the circulation faster than the rabbits.

Doping with Artificial Oxygen Carriers

There is a long history of science seeking to develop artificial substitutes for body parts damaged by disease or trauma. While defective teeth and limbs are commonly replaced by imitations without major loss of functionality, the development of a substitute for red blood cells has proved elusive. There is a permanent shortage of donor blood in western societies. Nevertheless, despite whole blood transfusions carrying measurable risks due to immunogenicity and the transmission of blood-borne infectious diseases, red blood cells are still relatively inexpensive, well tolerated and widely available. Researchers seeking to develop products that are able to meet and perhaps exceed these criteria have responded to this difficult challenge by adopting many different approaches. Work has focussed on two classes of substances: modified haemoglobin solutions and perfluorocarbon emulsions. Other approaches include the creation of artificial red cells, where haemoglobin and supporting enzyme systems are encapsulated into liposomes. Haemoglobin is ideally suited to oxygen transport when encased by the red cell membrane; however, once removed, it rapidly dissociates into dimers and is cleared by the kidney. Therefore, it must be stabilised before it can be safely re-infused into humans. Modifications concomitantly alter the vascular half-life, oxygen affinity and hypertensive characteristics of raw haemoglobin, which can be sourced from outdated blood stores, genetically-engineered Escherichia coli or even bovine herds. In contrast, perfluorocarbons are entirely synthetic molecules that are capable of dissolving oxygen but biologically inert. Since they dissolve rather than bind oxygen, their capacity to serve as a blood substitute is determined principally by the oxygen pressure gradients in the lung and at the target tissue. Blood substitutes have important potential areas of clinical application including red cell replacement during surgery, emergency resuscitation of traumatic blood loss, oxygen therapeutic applications in radiography (oxygenation of tumour cells is beneficial to the effect of certain chemotherapeutic agents), other medical applications such as organ preservation, and finally to meet the requirements of patients who cannot receive donor blood because of religious beliefs. Given the elite athlete's historical propensity to experiment with novel doping strategies, it is likely that the burgeoning field of artificial oxygen carriers has already attracted their attention. Scientific data concerning the performance benefits associated with blood substitutes are virtually nonexistent; however, international sporting federations have been commendably proactive in adding this category to their banned substance lists. The current situation is vulnerable to exploitation by immoral athletes since there is still no accepted methodology to test for the presence of artificial oxygen carriers.